1. Field of the Invention
This invention relates to a dental radiographic apparatus for obtaining an X-ray photographic picutre of a curved plane section of the entire jaws of a patient.
2. Prior Art
The dental radiographic apparatus for photographing the entire jaws comprises an arm rotatably mounted on a support, an X-ray generator mounted at one end of the arm, and a film holder mounted at the other end of the arm in an opposed relation with the X-ray generator, and is designed to take a tomographic picture of the curved plane of the entire jaws of a patient by irradiating X-ray beams from the generator upon the film surface in the film holder while rotating the generator and the film holder around the head of the patient by rotating the arm. Since the tomographing of the curved plane of the entire jaws conducted by the apparatus of the type described makes it possible to provide a panoramic picture of the entire jaws to enable overall inspection of the entire jaws at a glance, the apparatus has become indispensable for the overall examination not only of the individual tooth but also of the entire jaws.
However, although it is of course necessary that the radiographic image of the entire jaws be clear and vivid, the apparatuses heretofore in use have many problems to be solved. One of the conventional apparatuses is designed such that the rotation center of the arm is stepwise changed during the rotation to thereby make the film holder rotate, describing an elliptical locus composed of three circles. However, since such apparatus made it necessary to change the rotation center of the arm three times while photographing the apparatus involved a problem to make a photographic image unclear before and after such changes. In an attempt to solve the problem of the kind described, the present applicant earlier provied a radiographic apparatus (Japanese patent application No. 53165/1972) constructed in such a manner that a film holder is smoothly rotated tracing an elliptical locus by forming a cross groove on a support, providing the arm with two projections, and continuously changing the rotation center of the arm while sliding the two projections in the cross groove in accordance with the rotation of the arm. The apparatus had an advantage in that the same can satisfactorily solve the problem of the kind described, while on the other hand, the apparatus had a disadvantage in that the teeth contiguous to each other are photographed in a double image on the portion where X-ray beams from the X-ray generator fell slantly upon the dental arch and had another disadvantage in that the image of each tooth is different in enlargement ratio.
In view of the above disadvantages, the applicant later filed a patent application for a radiographic apparatus (Japanese patent application No. 97303/1974) constructed in such a manner that the support is provided with an arcuate groove and a linear groove passing through the center of the arcuate groove and the arm is provided with the aforestated two projections and a film holder can be rotated smoothly describing an elliptical locus and X-ray beams may fall on any point of the dental arch at right angles with the dental arch. Since the apparatus made it possible for the X-ray beams to fall on the dental arch intersecting the dental arch at right angles, the apparatus was enabled to remove the disadvantage of a double image of the teeth contiguous to each other but nevertheless there still remained such drawback that an arm driving roller slipped sideways in its rotation with respect to the support to render the movement of the arm instable.